Highly densified and machinable tungsten-iron-nickel alloy



United States Patent Ofiice 3,241,955 Patented Mar. 22, 1966 3,241,955 HIGHLY DENSIFIED AND MACH-UNABLE TUNGSTEN-IRON-NHCKEL ALLOY Arthur C. Neeley, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed May 6, 1963, Ser. No. 278,498

3 Claims. (Cl. 75-176) This invention relates to novel tungsten-base alloys and more particularly to improved tungsten-iron-nickel alloys exhibiting unusually high sintered densities combined With excellent machinability.

Because of its unique combination of physical and nuclear properties, the element tungsten is an excellent candidate material for use as a gamma ray shield in stationary and mobile applications. In practice, tungstenbase alloys, rather than the element, are employed in an effort to obtain full density, introduce a useful measure of ductility and obtain the benefit of increased structural strength of the alloy in comparison to the brittle tungsten metal. To manufacture such alloys resort must be had, as a practical matter, to powder metallurgical techniques in which the required elements, in powdered form, are pressed into a geometry of desired shape and subsequently sintered to synthesize the final alloy.

It is known, in general terms, that many desirable physical properties which one attempts to impart to a sintered alloy such as ductility, tensile strength, compressive and fatigue strength, tend to increase with increasing density, reaching maximum values as the sintered density approaches the theoretical density of the particular alloy. However, sintered compacts, even when sintered under ideal conditions, are inherently porous, so that notwithstanding any judicious selection of the alloy components, the maximum benefits realizable by such selection may be adversely affected when the selected alloy is synthesized by powder metallurgical techniques. The adverse effects of porosity are particularly relevant when maximum density is, in and of itself, a necessary quality of the alloy. It is therefore an object of the present invention to provide a tungsten-base alloy which can be synthesized to a maximum sintered density.

Because it is so brittle, tungsten and many tungsten base alloys are frequently diflicult to machine. It is therefore an additional object of this invention to provide a novel tungsten-base alloy which is not only sintered to a maximum density, but is also machina-ble to precision tolerances. As that term is used herein, machinability refers to the ability of a reasonably skilled machinist to machine the tungsten-base alloys of this invention by single point machining operations using a carbide, diamond or sapphire cutting tool and/ or the ability to drill and tap holes cleanly in parts made of the subject alloys without unduly distorting or fracturing the part being machined.

A further object of this invention is to provide a machinable tungsten-base alloy containing in excess of 98.5 tungsten, by weight.

These and other objects are met in accordance with this invention by a tungsten-base alloy containing as essential alloying ingredients 0.3 to 0.5 nickel Weight percent, 0.8 to 1.1 weight percent iron and the remainder tungsten.

The alloys of this invention are synthesized by standard powder metallurgical techniques in which finely divided, highly purified metal powders in the required proportions and having an average particle size in the range 1 to 8 microns are blended and cold pressed at a pressure in the approximate range 22,500 p.s.i. to 30,000 p.s.i. to reach a green strength equivalent to about 50% of the theoretical density of the alloy to be synthesized. The

resulting green strength composite is then sintered in a hydrogen atmosphere at a temperature at which a liquidus forms between the nickel and iron components of the alloy. A liquidus phase of this character will form with the subject alloy at a temperature in the range 1445 C. to 1450 C. The time at sintering temperature necessary to effect maximum sintering density will usually require from about one-half to two hours depending upon cross-sectional area with further sintering having virtually no effect on the attained sintered density.

The alloys of this invention can be sintered according to this procedure to within at least 98.5% of their theoretical density and, in each case, will be found to be effectively machinable as that term is herein defined. The remarkably high attained sintered densities appear to be a function of the iron content of the alloy. The effect of the iron content will be seen from an examination of the following table which lists the composition and attained sintered densities and theoretical density of various tungsten-iron-nickel alloys formed in accordance with the general procedure described above. For comparison purposes, the table also includes tungsten-base alloys having compositional limits which fall outside the scope of this invention.

The eflect of iron content upon density attainment of the high density tungsten alloy Alloy (Content, Sintered density (g./cc.) Percent of wt. percent) theoretical Alloy N 0. density attained Fe N1 W Attained Theoretical The remarkable dependency of the attained sintered density on the iron content can be seen from a comparison of alloys 1, 6, 11, 16, 21 and 26. It will be noticed that in this sequence of alloys a reduction in the highest density material, tungsten of 0.5% resulted in a lowering of the theoretical density from 19.03 to 18.89. Yet, in these alloys, at constant nickel content and decreasing tungsten content, there resulted an increase in the attained sintered density from 18.34 to 18.79-amounting to an increase in the attained density of 2.2%. This apparent anomalous behavior of increased attained density despite a decrease in the highest density alloy constituent may be explainable by an examination of the phase relationships involved in this alloy system. There exist two identifiable phases: a substantially continuous matrix phase consisting of a solid solution of major amounts of nickel and iron and a minor amount of tungsten dissolved i n i therein and a precipitated randomly oriented tungsten grain structure dispersed within the matrix phase, said precipitated phase containing an equilibrium composition consisting of 98.9% tungsten, 0.8% iron and 0.3% nickel. It appears that, within the iron/ nickel concentration ratio limits of this invention, the matrix volume is maintained at a maximum to thereby decrease the porosity of the highly sintered alloy to a minimum and thus closely approach the theoretical density of the alloy.

The upper and lower concentration limits are determined by the objective of obtaining a highly densified as well as machinable alloy. The lowest nickel and iron concentration limits are particularly sensitive with respect to machinability. Thus, consider an alloy consisting of 0.8 weight percent iron, 0.3 Weight percent nickel and the remainder tungsten. This alloy is machinable and a photomicrograph of its phase structure will show an essentially equiaxed tungsten grain structure with rounded edges and a considerable matrix volume with relatively no voids, i.e. porosity. A decrease of as little as only 0.1% in nickel and 0.1% in iron changes the resulting alloy to one which is essentially non-machinable although said alloy may still have a desirably high attained sintered density. The machinability of the alloys of this invention at the upper iron and nickel concentration limits of the subject alloy system is not as sensitive as the lower iron and nickel limits. Thus, an alloy consisting of 1.5 weight percent iron, 0.5 weight percent nickel and 98 weight percent tungsten will still be machinable in the sense described herein. However, the attained sintered density will have been reduced, thus reducing the desirable physical properties concomitant with maximum attained sintered density. A photomicrograph of an alloy consisting of 1.1 Weight percent iron, 0.5 weight percent nickel and 98.4 Weight percent tungsten shows a tungsten grain structure with rounded edges and a visible matrix with virtually no porosity; whereas, an alloy consisting of 1.5 weight percent iron, 0.5 weight percent nickel and 98 weight percent tungsten will show a similar tungsten grain structure With an excessive increase in matrix volume thus adversely lowering the attained density. Tungsten alloys containing from 0.8 to 1.1 weight percent iron but less than 0.3 weight percent nickel are too brittle to machine in the sense defined here. An increase in iron or nickel content above the lower concentration limits will increase the volume of available matrix and hence increase the attained sintered density. An increase in machinability will be more evident with increasing nickel content.

I have described a tungsten-base alloy containing an excess of 98.5% tungsten which is characterized by an attained sintered density which reaches as much as 99.6% of the theoretical density and which is machinable within concentration limits hereinbefore defined. The utility of such an alloy in various shielding applications has already been explained. However, this is not intended to limit its area of utility for this alloy and its unique combination of physical qualities may be used to advantage wherever high density structural material is needed which can carry a heavy load and which may require it to be subject to certain machining operations of the kind hereinbefore described.

It should be understood that a recital of the subject alloy in terms of consisting essentially of certain proportions or range of proportions refers to those alloy constituents which are deliberately added to determine the novel properties of said alloy, and that in a given case, an alloy of this invention may include unspecified impurities which do not detract from such properties. Carbon 0r graphite is an example of an impurity which detracts from the desirable properties of the claimed alloy since its existence, even in amounts as low as 500 p.p.m. may promote the formation of extremely brittle carbides such as tungsten carbide and should be scrupulously avoided during the synthesis of said alloy.

Having thus described my invention, I claim:

1. A tungsten-base alloy consisting essentially of 0.3 to 0.5% by weight nickel, 0.8 to 1.1% by weight iron and the remainder tungsten.

2. A machinable sintered tungsten-base alloy consisting essentially of 0.3 to 0.5% by weight nickel, 0.8 to 1.1% by Weight iron and the remainder tungsten, said alloy being further characterized in having a structure comprising a matrix phase comprising a solid solution of nickel, iron and tungsten W and precipitated W grain structure dispersed within said matrix having an equilibrium composition consisting essentially of 98.9% tungsten, 0.8% iron and 0.3% nickel, all by weight, said alloy being further characterized in that its attained density is within at least 98.5 of its theoretical density.

3. The alloy of claim 2 characterized in that it has a sintered density of at least 18.7 g./ cc.

References Cited by the Examiner UNITED STATES PATENTS 1,496,457 6/1924 Fonda -176 2,227,445 1/1941 Driggs 75--176 2,470,790 5/1949 Price 75-l76 3,177,076 4/1965 Timmons et a1. 75-176 LEON D. ROSDOL, Primary Examiner.

REUBEN EPSTEIN, CARL D. QUARFORTH,

Examiners.

R. L. GOLDBERG, R. L. GRUDZIECKI,

Assistant Examiners. 

1. A TUNGSTEN-BASE ALLOY CONSISTING ESSENTIALLY OF 0.3 TO 0.5% BY WEIGHT NICKEL, 0.8 TO 1.1% BY WEIGHT IRON AND THE REMAINDER TUNGSTEN. 